There are many situations in which the administration of a whole blood suspension or a component thereof is therapeutic. The nature of the condition dictates which suspension is appropriate. For example, transfusions of red blood cells are useful in patients suffering from chronic anemia which can result from disorders such as kidney failure, malignancies, or gastrointestinal bleeding. In addition, red blood cells are useful in treating acute blood loss resulting from trauma or surgery. Platelets, which are part of the blood clotting system, are useful in treating thrombocytopenia, a condition in which there is a shortage of platelets. Platelets have also been found to be helpful in treating cancer patients. Finally, although plasma is not typically used in transfusions, it is fractionated into specific products including albumin, clotting factor concentrates, and intravenous immune globulin. These fractions can be administered to a patient in need thereof.
Whole blood is typically subjected to centrifugation to separate out the three main components: red blood cells, plasma, and platelets. However, centrifugation does not provide complete separation. For example, a red blood cell suspension that has been centrifuged from whole blood will typically contain a portion of the leukocytes, some platelets, fibrinogen, fibrin strands, small fat globules, as well as small amounts of other blood components.
Regardless of the particular fraction or component of whole blood that is used, there is general agreement that the presence of white blood cells in the component is undesirable. It is believed that transfusion of blood components in which white blood cells are present has been responsible for adverse reactions ranging from a mild reaction, such as fever, to death from Graft-Versus-Host disease, in which the transfused leukocytes cause irreversible damage to the recipient's organs including the skin, gastrointestinal tract and neurological system. Thus, efficient removal of white blood cells from whole blood and blood components is needed.
To address this need, a number of materials and methods have been developed to remove white blood cells from a blood component. However, removal is not a straightforward matter of simple filtration. There are a variety of white blood cells or leukocytes including granulocytes, macrocytes, and lymphocytes. While the granulocytes and macrocytes are similar in size (≧15 microns in diameter), the lymphocytes are considerably smaller (between about 5-7 microns in diameter) and are also comparable in size to red blood cells (about 7 microns in diameter). This disparity in cell size makes simple filtration of leukocytes difficult.
In addition, a red blood cell suspension is typically contaminated with other components such as microaggregates and platelets. Microaggregates, which are small aggregates of components such as red blood cells, leukocytes and platelets, occur in blood that has been stored for extended periods of time, and can be up to 200 microns in diameter. Platelets, although smaller in size than micro aggregates, tend to be adhesive. The microaggregates and platelets can collect on or within pores of a simple filter and can cause clogging thereof. Due to the above, there is a present need for a product that is able to efficiently remove leukocytes without becoming clogged with microaggregates and the like.
Typically, when blood is drawn from a donor, approximately 450 ml of whole blood is supplied into a bag which usually contains an anticoagulant to prevent the blood from clotting. Hereinafter, this quantity of a whole blood sample will be referred to as a unit or a unit of whole blood. As detailed above, whole blood is rarely used in ibis form. Rather, most units are processed by centrifugation, gravity settling, or otherwise, to produce a unit of red cell concentrate in blood plasma known as packed red cells (PRC).
The volume of a unit of PRC varies considerably dependent on the hematocrit (percent by volume of red cells) of the drawn blood which usually falls within the range of from 37% to 54%. The hematocrit of the PRC usually falls within the range of from 70% to 80%. As a result, most units of PRC fall within the range of from 250 to 300 ml. However, variations below and above these figures are not uncommon.
For the purposes of convenience, as well as to more safely maintain the sterility of the blood sample, it is desirable for a leukocyte removal device to efficiently remove the leukocytes from a unit of whole blood or a unit of PRC. Thus, it is desirable for leukocyte removal device to efficiently remove leukocytes from about 500 ml of whole blood or 250-300 ml of PRC. Furthermore, in order to minimize the detrimental effects of the leukocytes, efficient removal in excess of 98% of each of the different types of leukocytes is desirable.
Leukocyte Separation
U.S. Pat. No. 4,925,572 to Pall discloses filtration devices using fibrous, non-woven media to capture leukocytes from a blood suspension. The device includes a series of porous elements having successively smaller pore diameters. The porous elements include synthetic resins such as polyvinylidene fluoride, polyethylene, polypropylene, cellulose acetate, nylon 6 and 66, polyester, polyacrylonitrile and polyaramid. These polymers are formed into fibers using a melt blowing process. However, the polymeric filtering media formed in this fashion have a surface tension which resists flow of liquid into the filtering media. The media may then require modification by surface grafting monomers onto the formed polymer in order to improve the wetting characteristic.
U.S. Pat. No. 5,820,755 to Kraus et al. discloses a filter unit and method of removing leukocytes which includes a commercially available nitrocellulose membrane having a pore size of 5-15 microns. The filter unit includes a plurality of layers, at least two of which are non-woven fibrous material as prefilter elements and one or more which is the nitrocellulose membrane. The filters may optionally be chemically modified by performing a surface grafting reaction with a monomer. Flow rates determined for various configurations of membrane diameters and pore sizes and ranged from 0.5 ml/min using fresh and aged blood samples. The flow rates were in the range of about 0.5 ml/min to 50 ml/min and diminished as prefilters were added to remove larger particles in the aged blood. It appears that the flow rate and ability to filter larger amounts of blood are reduced when this method is used on aged blood. Another disadvantage of this invention is the complexity of design which requires at least three separate elements to perform the separation function.
European Patent Application 155,003 to Asahi discloses a packed column having a main filter of fibers having an average diameter of from 0.3 microns to less than 3 microns, a bulk density of from 0.01 g/cm3 to 0.7 g/cm3, and an average distance between two adjacent filters defined by an equation whereby the average distance is in the range of from 0.5 microns to 7.0 microns. The fibers are selected from a variety of synthetic fibers such as polyesters, polypropylenes, and the like, to natural fibers. The fibers are made by blowing air or high-pressured steam on a mass of fibers and bonding them to one another. Alternatively, the fibers may be entangled and bonded with either heat or an adhesive. The fibers are oriented perpendicular to the direction of blood flow. A prefilter may optionally be used on blood having microaggregates.
European Patent Application 0 406 485 A1 discloses a filter unit and a method of removing leukocytes which includes a non-fibrous continuous pore matrix foam or membrane as the filter. The matrix has a distribution of pore sizes along a gradient. On the upstream side the element has larger pores and on the downstream side the element has smaller pores. As in the aforementioned filter units, this filter also has a randomized pattern of fiber distribution and pores, albeit with a non-randomized porosity gradient from large to small in the direction of flow of the suspension.
A disadvantage of each of the aforementioned filters is that they possess a randomized pattern of fiber distribution or foam porosity which includes a randomized pattern of pore channels. This arrangement can have the effect of reducing the flow rate of blood through the filter without having an associated improvement in removal efficiency. Furthermore, this configuration is more likely to cause clogging of the filter with a reduced capacity with respect to the total amount of blood that may be separated using the filter.
Methods of Making Porous Polymeric Materials
Conventional methods are known for use in making a variety of biocompatible polymeric materials such as the aforementioned leukocyte blood filtering materials. In addition to the methods listed above, the conventional methods also include solvent casting, particulate leaching, melt molding, phase separation, in situ polymerization, and membrane lamination. Thomson, R. et al., “Polymer Scaffold Processing,” Principles of Tissue Engineering, Lanza R. et al., eds., R.G. Landes Co. (1997).
In solvent casting, salt particles are dispersed in a polymer solution, for example a poly(L-lactic acid)/chloroform solution and cast into a container. The salt is insoluble in the solvent and forms crystal structures in the solution. The solvent is evaporated and the salt is removed from the resultant polymer layer by immersion in water or a heat treatment. The pores are the spaces that were filled by the salt. Membranes formed in this manner may attain porosity of up to about 93% and possess inter-connected pore channels. These membranes, however, are two dimensional and must be laminated, for example by application of additional chloroform, in order to form a three dimensional structure.
Foams and sponges are commonly formed from synthetic or natural polymeric materials using phase separation. In particular, phase separation upon freeze-drying has been used extensively. The polymeric materials are dissolved in a suitable solvent and rapidly frozen. The solution is then freeze-dried which removes the solvent and leaves behind a porous structure. Materials fabricated from natural polymers using this technique typically have average pore sizes ranging from 1-250 microns, depending on freezing conditions. However, in most applications where foams or sponges are helpful, the average pore size is preferably about 50-150 microns.
It is also known to form porous matrices by using the technique of fiber bonding. One such method uses poly(L-lactic acid) (“PLLA”) dissolved in methylene chloride to Loin a liquid which is cast over a non-woven mesh of poly(glycolic acid) (PGA) fibers. Methylene chloride is not a solvent for PGA. The solvent is then removed and the PLLA/PGA composite is heated to a temperature above the melting point of PGA. This allows the PGA to form bonds where the fibers overlap one another. The PLLA is then selectively dissolved in methylene chloride to remove it from the matrix. Mikos A. et al., “Preparation of poly(glycolic acid) bonded fiber structures for cell attachment and transplantation,” J. Biomed. Mater. Res., 27:183-89 (1993). In addition to being complex, this method has the disadvantage of using, in this case, a solvent known to be carcinogenic. Residual solvent left in the matrix could therefore pose a health hazard if it is subsequently released from the matrix into the treated blood sample. In addition, this technique does not lend itself to easy and independent control of porosity and pore size.
U.S. Pat. No. 4,475,972 to Wong discloses a porous polymeric material suitable for use as a vascular graft as well as a method of making same. The method involves extruding a viscous solution of a biocompatible polymer from a spinneret to form a plurality of filaments wound on a rotating mandrel. The contact of the wet filaments are points where the filaments become bound to one another after the solvent is evaporated. A thickness of 500 microns is achieved by approximately 800 passes of the spinneret over the mandrel. The pore size can be adjusted by varying the size of the fibers as well as the angle of the spinneret with respect to the mandrel known as the winding angle.
U.S. Pat. No. 6,056,993 to Leidner et al. discloses a porous tubular synthetic prosthesis and a method of making same. The method involves electrostatic spraying of both a water-soluble and a water-insoluble fibrous component onto a rotating mandrel or mold to form a prosthesis precursor. Next, to create porosity in the prosthesis, the water-soluble fibrous component is at least partially removed. The water-soluble fiber portion acts as a spacer, the removal of which creates porosity in the prosthesis. However, the method does not permit selection of the size, shape, location, or design of the pores forming the internal architecture of the prosthesis, because the electrostatic spray system involves a random application of fibers.
Each of the conventional methods form a material having characteristics and limitations which preclude use of the formed material in more than a few medical applications. Furthermore, the materials formed from conventional methods are often not optimal even for the few applications for which they have been designed. As a result, there is a present need for a material having structural and other parameters which make it suitable for a variety of medical applications. There is also a need for a method of making such a material.